Moons vs planets

  1. Hi all,

    I was just wondering whether there is a proper difference between a moon and a planet. The only answer I've been able to find is that moons orbit planets, and planets orbit stars. But in reality, planets and moons both orbit their common centre of mass. So to frame my question clearly, would it be a planet with a moon if they were the same mass? Would they both be planets, both be moons or what? Kind of a pedantic question perhaps, but I couldn't find a clear answer.
  2. jcsd
  3. mfb

    Staff: Mentor

    There is no clear definition. A possible distinction can be done based on the barycenter (the center of mass in the system): if it is within one object, you could call it planet and moon, otherwise it is more like a double planet.

    With that definition, pluto and charon are a double-minor planet (or double-minor planets?), and all 8 planets in our solar system are planets - even earth with its very large and distant moon.
  4. D H

    Staff: Mentor

    That is precisely the definition. It's important to remember that there are but eight planets in the entire universe per the IAU definition of the term "planet". Those things that orbit other stars: They aren't "planets". They're exoplanets.
  5. Yes, but that's only from OUR frame of reference. From THEIR frame of reference, we are the exoplanets :smile:
  6. mfb

    Staff: Mentor

    For our solar system alone, such a rule is not necessary - the earth/moon system will stay planet&moon anyway (try to change that :D), and all other planets have a huge mass difference between planets and moons as Pluto is not a planet any more.

    It will get more interesting for exoplanets as soon as exomoons are discovered.
  7. D H

    Staff: Mentor

    A rule is necessary. Is the Earth-Moon system a planet and satellite, or is it a double planet system? The IAU definition of a satellite is what says that the former is the case.

    The Moon may eventually recede to the point where the Earth becomes tidally locked to the Moon. At this point, will our Moon have become something other than a moon? (i.e., will the barycenter eventually migrate to being outside the Earth?) The answer is no because tidal locking will occur when the Moon recedes by another 35%. The barycenter will remain inside the Earth.
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